S.H. Gee

Synthesis of nano-sized spherical barium-strontium ferrite particles

Magnetic recording media requires good particle dispersion, a smooth surface, and small interparticle interaction to make an adequate signal-to-noise ratio (SNR). Well dispersed 50-60 nm sized spherical barium-strontium ferrite (S-Ba/Sr-Fe) nanoparticles were successfully prepared with 40 nm sized hematite precursor particles and BaCO/sub 3//SrCO/sub 3/ colloid. The coercivity and saturation magnetizations of S-Ba/Sr-Fe nano-particles were 1568 Oe and 48.6 emu/g, respectively. In order to evaluate magnetic interaction, magnetic tape was prepared using an Eiger mill with binder and organic solvent. /spl Delta/M measurement showed the S-Ba/Sr-Fe nanoparticles in the tape had negative magnetic particle-to-particle interaction.

Microstructure and magnetic properties of hexagonal barium ferrite thin films with various underlayers

BaM (barium ferrite) thin films and underlayers (Fe, Cr, Al2O3, Fe2O3, ZnFe2O4, TiO2) were prepared by rf/dc magnetron sputtering on (100) oriented bare Si substrates. The effects of the underlayer on grain orientation, magnetic properties, and microtexture of BaM film were studied. All the BaM films, except BaM/Fe/Si film, attained nearly the same perpendicular and in-plane coercivities. The BaM/TiO2/Si exhibits the highest coercivity. However, regardless of the underlayer, BaM grains are randomly oriented. By adopting ZnFe2O4 as an underlayer, the interdiffusion of Si from substrate was prohibited to some degree. Elongated grains from the extinction of small platelet grains were grown with an increase in the BaM film thickness of BaM/Si. The microstructure of BaM in BaM/TiO2/Si was strongly dependent on both the microstructure of TiO2 underlayer and the total sputtering gas pressure.

Ba

A combination of energetic shake-milling and a subsequent double sintering process was employed to synthesize Co_0.8Zn_1.2Z (Ba₃Co_0.8Zn_1.2Fe₂₄O ₄₁) hexaferrite nanoparticles with a high-saturation magnetization and a low coercivity. A homogeneous mixture of BaCO₃, CoO, ZnO, 40-nm sized alpha-Fe₂O₃, and heat-treating in an oxygen environment were important factors for synthesizing single-phase Co₂Z-type ferrite nanoparticles. In addition to an X-ray diffraction pattern, Mossbauer spectra confirmed that only Fe³⁺ cations are present in the synthesized Co_0.8Zn_1.2Z particles, implying single phase of the particles. Low-temperature sintering processing, 900degC, was then successfully applied to the single-phase Co_0.8Zn_1.2Z particles. The coercivities of the Co _0.8Zn_1.2Z powder and the low-temperature sintered disk were 9 and 20 Oe, respectively, while maintaining the saturation magnetization of 50 emu/g

_3

Liquid phase epitaxy technique was used to grow 144 μm thick barium ferrite (BaFe12O19; BaM) single crystalline films on (111) Gd3Ga5O12 substrate. The growth rate of 72 μm/h was achieved with a flux system of Fe2O3–BaCO3–Na2CO3. The grown BaM films show single crystalline (000l) orientation that was confirmed by x-ray diffraction and magnetic torque curves. The saturation magnetization (4πMs) and the anisotropy field (Hk) were found to be 4.2 kG and 16.0 kOe, respectively. The ferrimagnetic resonance linewidth (ΔH) at 35 GHz was measured to be 0.1 kOe.

Co

Magnetization configuration and magnetic switching behavior in two types, P-I and P-II, of submicrometer Pac-man-shaped Ni/sub 80/Fe/sub 20/ magnetic elements are studied using both magnetic force microscopy (MFM) and numerical simulation. It was found that a slight variation in the shape of the elements has a striking influence on the internal magnetic structures and switching field distribution. In particular, the vortex-formation-driven switching is replaced by quasi-coherent reversal by removing the central core part at the center of element. The sensitive dependence of remanent magnetic configuration and switching behavior on sample geometry is discussed in terms of the competition between the exchange and demagnetizing energy terms.

_0.8

We report 24-30 nm spherical barium ferrite (S-BaFe) particles having extremely narrow size distribution without any superparamagnetic phase. We have converted spherical magnetite (S-Mag) nanoparticles to S-BaFe nanoparticles using a unique adsorption-diffusion process. The synthesized S-BaFe nanoparticles were characterized by X-ray diffractometer, Mossbauer spectrometer, transmission electron microscope (TEM), and vibrating magnetometer (VSM) for magnetic and physical properties. Saturation magnetization and coercivity of the S-BaFe nanoparticles were found to be 41.4 emu/g and 4075 Oe, respectively. The thermal stability of Ku V/kB T ap 107 was estimated for the S-BaFe nanoparticles from time-dependent remanent coercivity measurement.

Zn

Monodisperse hematite (α-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a ∼ 0.50327 and c ∼ 1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of ±45° revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263 K of bulk hematite. It was ascertained from the previous Mossbauer studies that the spin orientation for nano-sized hemat...

_1.2

Interactions between neighboring cells become increasingly important due to the miniaturization of magnetoelectronic devices. This paper studies the effect of magnetic interaction on the switching behaviors in two different configurations of paired Pac-man shape Permalloy elements: back-to-back and face-to-face configurations. From as-patterned state MFM images, it is observed that the face-to-face configuration is prone to form either two single domains with an antiferromagnetic configuration, one single domain with one vortex or a double vortex configuration. MOKE hysteresis loops show that the coercivity for the face-to-face configuration is smaller than the back-to-back configuration. These experimental results indicate that the back-to-back configuration has weaker interaction between the two Pac-man elements than the face-to-face configuration. We further varied the aspect ratio of Pac-man elements in the pair arrays to tune the magnetic interaction. It was found that the coercivity of pair array increased with the higher cell aspect ratio. Micromagnetic simulation was also performed to simulate the switching process for the two different configurations. Overall, the back-to-back configuration is recommended for applications that demands less inter-cell interactions.

Fe

The lithographic requirements for the thin film head industry are comparable to the semiconductor industry for certain parameters such as resolution and pattern repeatability. In other aspects such as throughput and defectivity, the requirements tend to be more relaxed. These requirements match well with the strengths and weaknesses reported concerning nanoimprint lithography (NIL) and suggest an alternative approach to optical lithography. We have demonstrated the proof of concept of using NIL patterning, in particular Jet and FlashTM Imprint Lithography (J-FILTM) 1 , to build functional thin film head devices with performance comparable to standard wafer processing techniques. An ImprioTM 300 tool from Molecular Imprints, Inc. (MII) was modified to process the AlTiC ceramic wafers commonly used in the thin film head industry. Templates were produced using commercially viable photomask manufacturing processes and the AlTiC wafer process flow was successfully modified to support NIL processing. Future work is identified to further improve lithographic performance including residual layer thickness uniformity, wafer topography, NIL→NIL overlay, and development of a large imprint field that exceeds what is available in optical lithography.

_24

We investigated spin-polarized current switching of elongated Pac-man (EPM) elements in a Pac-man shaped spin valve (Co/Cu/Py). The aspect ratio, pulse duration, and effect of antiferromagnetic (AFM) layer were simulated to obtain coherent switching. Pulse duration was varied on the picosecond (ps) scale and showed that ultra-fast switching could be achieved. A critical aspect ratio of 4.2 was found, where a minimum current density was observed. For antiparallel to parallel (AP-P) switching, a vortex formed during the switching process for aspect ratios less than 7. Aspect ratios of 7 or higher did not form a vortex and showed similar current densities to parallel to antiparallel (P-AP) switching as a result. It was found that the AFM layer prevented the formation of a vortex at the critical aspect ratio of 4.2 and effectively changed the switching mechanism. This allowed coherent switching with current densities for AP-P and P-AP to be only 7% from each other at the aspect ratio of 4.2.